pwm: pca9685: Support new usage_power setting in PWM state

If usage_power is set, the pca9685 driver will phase shift the
individual channels relative to their channel number. This improves EMI
because the enabled channels no longer turn on at the same time, while
still maintaining the configured duty cycle / power output.

Signed-off-by: Clemens Gruber <clemens.gruber@pqgruber.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>